On 1/16/23 06:13, Yu Zhao wrote:
On Sun, Jan 15, 2023 at 6:47 AM msizanoen1 <msizanoen@xxxxxxxxxxx> wrote:
In some cases, memory cgroup migration can be initiated by userspace
right after a process was created and right before `lru_gen_add_mm()` is
called (e.g. by some program watching a cgroup and moving away any
processes it detects[1]), which results in the following sequence of
WARNs followed by an Oops as the kernel attempts to perform a
`lru_gen_add_mm()` twice on the same `mm`:
...
Fix this by simply leaving the lru_gen alone if it has not been
associated with a memcg yet, as it should eventually be assigned to the
right cgroup anyway.
[1]: https://gitlab.freedesktop.org/benzea/uresourced/-/blob/master/cgroupify/cgroupify.c
v2:
Added stable cc tags
Signed-off-by: N/A (patch should not be copyrightable)
Cc: stable@xxxxxxxxxxxxxxx
Thanks for the fix. Cc'ing stable is the right thing to do. The
commit message and the comment styles could be easily adjusted to
align with the guidelines.
I don't think the N/A is acceptible though. I fully respect it if you
wish to remain anonymous -- I can send a similar fix crediting you
as the "anonymous user <msizanoen@xxxxxxxxxxx>" who reported this bug.
Sure, just add my email in the `Reported-by: ` and `Tested-by: ` lines
and git-send-email should automatically add me to the Cc list.
A bit of background on how I broke it: an old version I have on 4.15
calls lru_gen_add_mm() before cgroup_post_fork(), which excludes
cgroup migrations by cgroup_threadgroup_rwsem. When I rebased it, I
made lru_gen_add_mm() depend on task_lock for the synchronization with
cgroup migrations -- the decoupling seemed (still seems) to make it
less complicated -- but this is not safe unless we have the check below.
---
mm/vmscan.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index bd6637fcd8f9..0cac40e7484c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3323,13 +3323,19 @@ void lru_gen_migrate_mm(struct mm_struct *mm)
if (mem_cgroup_disabled())
return;
+ /* This could happen if cgroup migration is invoked before the process
+ * lru_gen is associated with a memcg (e.g. during process creation).
+ * Simply ignore it in this case as the lru_gen will get assigned the
+ * right cgroup later. */
+ if (!mm->lru_gen.memcg)
+ return;
+
rcu_read_lock();
memcg = mem_cgroup_from_task(task);
rcu_read_unlock();
if (memcg == mm->lru_gen.memcg)
return;
- VM_WARN_ON_ONCE(!mm->lru_gen.memcg);
VM_WARN_ON_ONCE(list_empty(&mm->lru_gen.list));
lru_gen_del_mm(mm);
